The invention relates to a high-pressure discharge lamp having a discharge device which is energizable for emitting visible light and ultraviolet radiation and which is surrounded within a light transmissive outer envelope by an inner and outer glass sheath.
Such a lamp is described in the prior Pat. Application No. NL 8900216. light which corresponds to U.S. application Ser. No. 458,112 filed Dec. 28, 1989. The glass sheaths are cylindrical and have their adjacent ends closed by a respective metal plate Here the glass sheaths serve to protect the environment of the lamp from the consequences of an explosion of the discharge device, which may occur at the end of the life of the lamp. The lamp is designed so that fragments of the discharge device and a glass sheath remain in the outer envelope due to the fact that the latter remains undamaged.
On discharge lamps of the said kind, which have a transparent outer envelope, i.e. an outer envelope not coated with powder, and which are intended to be operated in open luminaires, the requirement is imposed that they produce radiation which is not harmful for people and materials. Standards then hold with respect to:
the damage factor (Fd), which must be smaller than 0.25, where: ##EQU1## Herein, Cd=a constant; P(.lambda.)=the spectral power distribution; V(.lambda.)=the eye sensitivity curve and D(.lambda.)=the relative spectral damage function described by National Bureau of Standards (see Lighting Res. Techn. 20(2), 43-53, 1988)). PA1 the admissible irradiation time (PET), which for a 70 W lamp with an illumination intensity of 1000 lx must be larger than 16 hr (Nat. Inst. for Occupational Safety and Health), where ##EQU2## Herein Cp=a constant, P(.lambda.) and V(.lambda.) have the aforementioned meanings and S(.lambda.)=a function describing the relative effect of radiation on skin and eyes. PA1 The emitted UV-A power (P UV-A), which must be smaller than 0.55 W. PA1 the glass of the inner sheath has an SiO.sub.2 content of at least 96% by weight, PA1 the outer sheath consists of aluminosilicate glass, and PA1 the lamp vessel is surrounded by an interference filter reflecting UV radiation.
NL 8502966-A ) discloses a discharge lamp, in which the discharge is surrounded by an interference filter in that the lamp vessel (or discharge device) is covered with such a filter. However, the lamp emits a substantial quantity of UV-A radiation and also transmits UV-B and UV-C radiation Therefore, the lamp is intended to be used in a closed luminaire.
U.S. Pat. No. 4,281,274-A discloses a discharge lamp, which has around the lamp vessel (or discharge device) an open tube of borosilicate glass, which has a positive potential with respect to the lamp vessel. The tube of borosilicate glass, which would be opaque to UV radiation, must prevent that due to this radiation electrons are detached from metal parts of the lamp. Such electrons can be deposited on the lamp vessel and can give rise to loss of sodium from its filling. Nevertheless a positive potential is applied to the tube to collect and hold detached electrons.